The present invention relates to an electrolytic capacitor. More particularly, the present invention relates to a capacitor which has a low impedance, excellent low-temperature stability and heat resistance, and good working life characteristics.
A capacitor is a general electrical part and is widely used in power circuits and noise filters for digital circuits in various electric and electronic devices.
Various types of electrolytic capacitors are well-known at present, and examples thereof include aluminum electrolytic capacitors, wet tantalum electrolytic capacitors and the like. Among these electrolytic capacitors, particularly excellent effects are expected from an aluminum electrolytic capacitor in the practice of the present invention. Therefore, the present invention will now be described with reference to this kind of electrolytic capacitor. The term xe2x80x9celectrolytic capacitorxe2x80x9d used herein refers to an aluminum electrolytic capacitor, unless otherwise stated.
A conventional aluminum electrolytic capacitor can typically be produced by using an anode foil, which is made by etching a high-purity aluminum foil thereby to increase its surface area, and anodizing the surface of the aluminum foil to provide an oxidized film, and a cathode foil whose surface has only been etched. The resulting anode foil and cathode foil are disposed opposite each other and a separator (release paper) is interposed between those foils to form a laminate, and then an element (capacitor element) made by winding the laminate is impregnated with an electrolytic solution. The element impregnated with the electrolytic solution is contained in a case or casing (generally made of aluminum), which is then sealed with an elastic sealant, thus completing an electrolytic capacitor. The electrolytic capacitor also includes those other than those with a wound structure.
In the above-described electrolytic capacitor, the characteristics of the electrolytic solution may be a large factor which decides the performance of the electrolytic capacitor. With a size reduction of the electrolytic capacitor, an anode foil or cathode foil having a large surface area produced by etching has been used and the resistivity of the capacitor has recently increased. Therefore, an electrolytic solution having a low resistivity (specific resistance) and high conductivity is required as the electrolytic solution to be used in the electrolytic capacitor.
A conventional electrolytic solution for use in an electrolytic capacitor is generally prepared by dissolving as an electrolyte a carboxylic acid such as adipic acid, benzoic acid, etc. or an ammonium salt thereof into a solvent prepared by adding about 10% by weight or less of water to ethylene glycol (EG) as a principal solvent. Such an electrolytic solution has a specific resistance of about 1.5 xcexa9.m (150 xcexa9.cm).
On the other hand, the capacitor is always required to have a low impedance (Z) to sufficiently show the performance thereof. The impedance is determined by various factors and, for example, it is reduced when the electrode area of the capacitor increases. Therefore, the impedance can be reduced, of course, in case of a large-sized capacitor. An approach of reducing the impedance by improving a separator has also been made. However, the specific resistance of the electrolytic solution is a large controlling factor, particularly in a small-sized capacitor.
A low-specific resistance electrolytic solution has recently been developed using an aprotic organic solvent such as GBL (xcex3-butyrolactone) (see, Japanese Unexamined Patent Publication (Kokai) Nos. 62-145713, 62-145714 and 62-145715). However, the capacitor using this aprotic electrolytic solution is by far inferior in impedance in comparison to a solid capacitor using a known electronic conductor capable of affording a low specific resistance.
The aluminum electrolytic capacitor has poor low-temperature stability, because of use of the electrolytic solution, and a ratio of an impedance at xe2x88x9240xc2x0 C. to that at 20xc2x0 C. (100 kHz), Z (xe2x88x9240xc2x0 C.)/Z (20xc2x0 C.), is as large as about 40 at present. In addition, there arises a problem that, if water is contained in the electrolytic solution, it can freeze under the operation conditions at a low temperature. Under these circumstances, it is now required to provide an aluminum electrolytic capacitor which has a low impedance and excellent low-temperature stability.
Furthermore, water used as portion of the solvent in the electrolytic solution of the aluminum electrolytic capacitor is a chemically active substance to aluminum constituting the anode foil or cathode foil, and accordingly, there is a problem that water reacts with the anode foil or cathode foil, thereby to generate a hydrogen gas and to drastically deteriorate the characteristics.
On the other hand, in a conventional aluminum electrolytic capacitor, phenomena such as a gradual reduction in the capacitance of the capacitor and an increase in change of other capacitor characteristics are sometimes caused by formation of a hydrate film on the surface of the anode foil and cathode foil in a temperature accelerated test. These phenomena can cause a reduction in the working life of the electrolytic capacitor.
Since the above-described hydrate film is easily formed on the surface of the cathode foil which has not been anodized as compared with the anodized surface of the anode foil, the hydrate film may be caused by the fact that aluminum constituting the anode foil and cathode foil is an active metal. Therefore, a trial of adding an inhibitor for inhibiting formation of the hydrate film in the electrolytic solution, which is directly contacted with the anode film and cathode film, has been made. However, the inhibition effect dissipates and a satisfactory inhibition effect has still to be obtained. Accordingly, it has also been required to provide an electrolytic capacitor, particularly an aluminum electrolytic capacitor, capable of sufficiently preventing a hydrate film forming on the surface of the anode foil and cathode foil.
The present invention is directed to solve the above-mentioned problems of the prior art, and an object thereof is to provide an electrolytic capacitor which has a low impedance, excellent low-temperature stability and heat resistance, and a particularly long working life.
According to the present invention, the above object can be attained by an electrolytic capacitor comprising a capacitor element formed from an anode foil, a cathode foil opposed to the anode foil and a release paper sandwiched between the anode foil and the cathode foil, and an electrolytic solution, characterized in that:
the content of cation(s) in said release paper is not more than 500 ppm.
In general, the release paper used in the electrolytic capacitor contains metal salts (salts of an organic acid and an inorganic acid, such as carboxylic acid, sulfuric acid, and nitric acid) in a low concentration such as about several percent. That is, in the case where these metal salts and ionic compounds are contained in the release paper in a high concentration, when using the release paper in the electrolytic capacitor, metal salts contained in the release paper are eluted into the electrolytic solution in the form of ions, thereby to cause defects capable of exerting a deleterious influence on the capacitor characteristics, such as unstable conductive characteristics of the electrolytic solution and acceleration of the corrosion reaction.
The present inventors have studied currently and widely used release papers and have found that a vigorous reaction, between the electrolytic solution and electrode foils, under high-temperature conditions occurs when using a current release paper containing metal ions (cations) of Ca, Mg, Na, etc. in a concentration capable of exerting no influence on the conductive characteristics of the electrolytic solution, that is, the total amount of several thousand ppm to several percent.
Based on the above findings, the present inventors have further studied and found that drawbacks such as gradual reduction, with time, in the capacitance of the capacitor and large increase in change of the other capacitor characteristics can be prevented by reducing the content of cations in the release paper to a predetermined value or less. The xe2x80x9cpredetermined value of cationsxe2x80x9d used herein means the above content of 500 ppm.
The present inventors have also found that the content of cations such as calcium ion in the release paper can be reduced to a level of 500 ppm or less by subjecting the paper to a washing treatment.
Furthermore, they have found that the effects derived from the reduction in content of cations is enhanced when the electrolytic solution used in the electrolytic capacitor consists of an organic solvent and water and the amount of water contained in the organic solvent is large.
Accordingly, the electrolytic solution used in the electrolytic capacitor preferably contains a solvent, containing a highly increased amount of water, which consists of 20 to 80% by weight of an organic solvent and 80 to 20% by weight of water, and at least one electrolyte selected from the group consisting of a carboxylic acid or a salt thereof and an inorganic acid or a salt thereof. The organic solvent to be used in this electrolytic solution is preferably a protic solvent, an aprotic solvent, or a mixture thereof.
The carboxylic acid or salt thereof to be used as the electrolyte is preferably selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, p-nitrobenzoic acid, salicylic acid, benzoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, maleic acid, phthalic acid, azelaic acid, citric acid and hydroxybutyric acid, and ammonium, sodium, potassium, amine and alkyl ammonium salts thereof. The inorganic acid or salt thereof which is also used as the electrolyte is selected from the group consisting of phosphoric acid, phosophorous acid, hydrophosphorous acid, boric acid, sulfamic acid, and ammonium, sodium, potassium, amine and alkyl ammonium salts thereof. These electrolytes may be used alone or in combination.
Preferably, the electrolytic solution of the present invention further contains at least one additive selected from the group consisting of:
(1) a chelate compound,
(2) saccharides,
(3) hydroxybenzyl alcohol and/or L-glutamic-diacetic acid or a salt thereof,
(4) a nitro compound, and
(5) gluconic acid and/or gluconic lactone.
The electrolytic capacitor of the present invention is preferably an aluminum electrolytic capacitor, and the aluminum electrolytic capacitor is more preferably formed by comprising:
a capacitor element formed by winding an anode foil consisting of an aluminum foil, with an anodized film on the surface of the aluminum foil, and a cathode foil made of the aluminum foil via a release paper so that the surfaces face each other;
an electrolytic solution;
a case containing the capacitor element and the electrolytic solution; and
an elastic sealant with which an opening portion of the case is sealed.